It would just be a way of telling or reminding a group of young female individuals that they are girls.
Is the -or in "flickor" always pronounced "ur" (that is, as in "flee-cur")? I mean, in all the feminine plurals that 'o' is more like an 'u'?
It’s usually pronounced roughly like the vowel in English ’book’, that is [ʊ]. Many people also pronounce -or and -er the same, and say it as if it were spelt flicker, then it’s pronounced with the [ɛ] vowel. In southern Sweden, both -er and -or are often pronounced with an å-sound, making it sound like flickår that is roughly with [ɔ].
Seems to be that a word ending in "a" (presumably feminine) switches the "a" for "or" to become plural, hence "flickor" and "kvinnor".
"Ni" is plural "you". It's when you say "you" to several people, not one. English is peculiar because it doesn't have different pronouns for singular and plural "you", except in the the US in the South, where people often say "y'all" (a shortening of "you all"). "Y'all" is pretty cool I think because it's a perfect English translation of "ni" in Swedish.
But ni is also 'they' so I think better translation would be 'they are girls', isnt it?